Contact opening metrology

ABSTRACT

A method for process monitoring includes receiving a sample having a first layer that is at least partially conductive and a second layer formed over the first layer, following production of contact openings in the second layer by an etch process, the contact openings including a plurality of test openings having different, respective transverse dimensions. A beam of charged particles is directed to irradiate the test openings. In response to the beam, at least one of a specimen current flowing through the first layer and a total yield of electrons emitted from a surface of the sample is measured, thus producing an etch indicator signal. The etch indicator signal is analyzed as a function of the transverse dimensions of the test openings so as to assess a characteristic of the etch process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/209,087, filed Jul. 30, 2002, which is assigned to theassignee of the present patent application and is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to microfabrication of holes andtrenches, including vias, damascene structures and the like, insemiconductor device manufacturing, and specifically to monitoring ofcontact holes produced on semiconductor wafers.

BACKGROUND OF THE INVENTION

Contact hole production is a common step in semiconductor integratedcircuit manufacturing. The contact holes are typically used to makeelectrical connections to a semiconductor or metal layer through anoverlying non-conducting (dielectric) layer, such as an oxide layer, orpartially-conductive layer. In order to produce contact holes, a layerof photoresist is deposited on the wafer surface. The photoresist isexposed to ultraviolet or other radiation, hardened and developed inorder to form a “mask” over the wafer, with openings at the locations ofthe contact holes. Then the wafer is transferred to an etch station toform the contact holes through the non-conducting layer down to thesemiconductor layer. The photoresist mask is then removed, and thecontact holes are filled with metal. A similar process is used inproducing trenches or vias in the wafer surface.

In order to ensure consistent device performance, the depth, width andbottom surface cleanliness of contact openings must be carefullycontrolled. (In the context of the present patent application and in theclaims, the term “contact openings” refers to all structures of the typedescribed above, including both contact holes, vias and trenches.Certain techniques for inspecting contact openings and monitoring theirproduction, however, are described by way of example with specificreference to contact holes.) Deviations in the dimensions of contactopenings can lead to variations in the contact resistance. Thesevariations can have a serious impact on device performance and can leadto loss of process yield. The manufacturing process must therefore becarefully monitored and controlled, in order to detect deviations information of contact openings as soon as they occur and to takecorrective action to avoid the loss of costly wafers in process.

It is known in the art to use a scanning electron microscope (SEM) toinspect contact holes and other contact openings. The principles of theSEM and its use in microanalysis of semiconductor device structures aredescribed, for example, by Yacobi et al., in Chapter 2 of Microanalysisof Solids (Plenum Press, New York, 1994), which is incorporated hereinby reference. Because contact holes are typically much deeper than theyare wide, a special high aspect ratio (HAR) imaging mode is used, asdescribed by Yacobi et al. Open contact holes, which reach down throughthe dielectric layer to the semiconductor below, appear bright in theimage, while closed holes, which do not fully expose the semiconductorlayer, are dim.

HAR techniques using a SEM are time-consuming and costly to implement,and they become impractical at very high aspect ratios (roughly >10),which are used in some integrated circuits, such as DRAM. They are alsonot capable of distinguishing between different types of blockage thatcan cause contact holes to be closed (for example, under-etching of theholes, as opposed to deposition of residues in the bottoms of theholes). Furthermore, HAR imaging techniques can generally be used onlyafter the photoresist mask has been cleaned from the wafer surface.Consequently, there is no possibility of continuing the etching processif it is discovered upon inspection that the contact holes have beenunderetched.

An alternative method for contact hole inspection is described by Yamadaet al., in “An In-Line Process Monitoring Method Using Electron BeamInduced Substrate Current,” in Microelectronics-Reliability 41:3 (March2001), pages 455-459, which is incorporated herein by reference. Thesubstrate current in an electron beam system, also known as the specimencurrent, absorbed current or compensation current, is defined as theabsorbed current that flows or would flow from the primary electron beamto ground (earth) via the specimen (i.e., via the wafer). In otherwords, the specimen current is equal to the difference between theprimary beam current (i.e., the current of electrons in the electronbeam that irradiates the specimen in the system) and the total yield ofelectrons emitted from the surface of the specimen due to secondary andbackscattered electrons (adjusted for any local charging effects or timeconstants). The specimen current can be either positive or negative,depending on whether the energy of the primary electron beam is in thepositive- or negative-charging domain of the specimen. (The phenomena ofpositive and negative charging by e-beam irradiation are described inthe above-mentioned reference by Yacobi et al.) Yamada et al. directedan electron beam at single contact holes and groups of holes in a SiO₂surface layer overlying a silicon substrate, and measured the resultantspecimen current. They found that the specimen current was a goodindicator of hole-bottom oxide thickness, as well as of the holediameter.

Yamada et al. describe further aspects of contact hole measurement inU.S. patent application Publication No. US 2002/0070738 A1, whosedisclosure is incorporated herein by reference. Semiconductor devicesare inspected by measuring the specimen current in an area of a samplehaving no contact holes as a background value, and comparing this valueto the current measured in the area of a hole. A current waveform isautomatically evaluated in order to determine whether the measurement isindicative of a defect of the device or of manufacturing equipment usedin producing the device.

SUMMARY OF THE INVENTION

The present invention provides improved methods and systems forassessing characteristics of contact openings and other openings formedin dielectric or partially-conductive layers on a sample, andparticularly for monitoring the quality of contact openings, such ascontact holes, as well as vias or trenches, in production ofsemiconductor devices.

In embodiments of the present invention, a charged particle beam,typically an electron beam, characterized by a primary beam current, isdirected to irradiate areas of a sample in which contact openings orother openings are etched. The etch state of the contact openings isdetermined by measuring an etch indicator signal generated by the samplein response to the electron beam. The etch indicator signal is typicallybased on the overall specimen current flowing through a single openingor an array of openings in the sample circuit. Alternatively oradditionally, the etch indicator signal may be based the total yield ofsecondary and backscattered electrons emitted from the surface of thesample. The specimen current can be deduced from the difference betweenthe primary beam current and the total electron yield. Although certainembodiments are described hereinbelow with specific reference tospecimen current, the principles of these embodiments may generally beadapted to work with etch indicator signals based on the total yield ofsecondary and backscattered electrons, instead of or in addition to thespecimen current.

The “etch state” or “etch quality” of contact openings, as used in thecontext of the present patent application and in the claims, refers toone or more of a set of characteristics of the contact or otheropenings. These characteristics generally include the remaining(residual) thickness of any dielectric material at the bottom of thehole, and they may also include the contact, trench or via depth and/orwidth, the presence or absence of a residue inside the openings, and anypunch through an etch stop layer to damage underlying copper in adamascene structure. The measurement of specimen current is indicativeof the diameter of the bottom of the contact hole, where contact is madewith the underlying layer, unlike HAR SEM imaging, which shows mainlythe diameter at the upper end of the contact hole. The diameter of thebottom of the contact hole is a critical dimension, in terms of itseffect on the resistance of the contact formed when the hole is filledwith metal.

The etch indicator signal may be measured using a narrow particle beam,to irradiate the area of a single contact opening. Alternatively, agroup of contact openings may be irradiated simultaneously, typicallyusing a defocused or rastered focused beam, to give an enhanced signal,as described in the above-mentioned U.S. patent application Ser. No.10/209,087.

In some embodiments of the present invention, a calibration procedure isused to determine an absolute threshold specimen current for a given,nominal contact hole diameter. The actual specimen current is thenmeasured by irradiating a number of different contact holes of thenominal diameter, distributed over the surface of the sample. If themeasured magnitude of the specimen current due to irradiation of thesample in the area of a given contact hole is equal to or greater thanthe threshold, the contact hole is considered to have been etchedsufficiently. If the measured specimen current is significantly lessthan the threshold, however, the contact hole is considered to beunderetched, indicating that a process fault may have occurred.

Another calibration procedure is used to determine a relative thresholdfor the specimen current. The relative threshold defines a maximalnon-uniformity of specimen current measurements, made on the same typeof contact hole in different locations on the sample. Variations in thespecimen current measured at the different locations that are greaterthan the threshold are considered to be indicative of a problem in themanufacturing process.

In some embodiments of the present invention, a test structure is formedon the sample, either on a wafer scribe line or in-die, for use inassessing process quality. The test structure can, for example, comprisean array of contact openings of varying transverse dimension, such ascontact holes of graduated diameter, with some openings that are widerthan the nominal diameter of functional contact holes to be formed onthe sample, and others that are narrower. (In the context of the presentpatent application and in the claims, “transverse” refers to a directionor dimension perpendicular to the depth dimension of a contact opening.)During etching of the sample, the depths of the contact holes willincrease at a rate that is roughly proportional to their diameters. Whenthe etch process is adjusted properly, the specimen current measured forthe test holes in the vicinity of the nominal diameter or greater shouldbe high, indicating complete etching of the dielectric. The specimencurrent measured for the small-diameter holes may be markedly lower,indicating incomplete etching of these holes. The test structure thusprovides a reading of the variation of etch depth as a function of holediameter. Changes in this reading may be used to detect incipientprocess defects such as underetching, before the defects become seriousenough to affect the quality of the functional contact holes.

As another example, the test structure may comprise both dense andsparse arrays of contact holes having the same diameter. The etch rateof contact holes often is a function of contact hole density, due to amicro-loading effect when contact holes are closely spaced. Thus, ingeneral, the etch rate is substantially lower in the dense contact holearrays than in the sparse arrays. The spacing of the contact holes indense and sparse arrays in the test structure is typically chosen torepresent limiting cases of actual contact hole spacing for in-diepatterns. Alternatively or additionally, the density of the contactholes can be determined from prior knowledge of the etch process window.Therefore, by measuring the etch indicator signal with respect tocontact holes in the dense and sparse arrays, it is possible to detectetch problems that may occur within in-die patterns due tomicro-loading.

In further embodiments of the present invention, novel testconfigurations are used to enhance the strength or sensitivity of theetch indicator signal for a given particle beam current and contact holesize. These test configurations are useful in particular to enhancesensitivity to very thin layers or remaining dielectric at the bottom ofthe contact hole. In one of these embodiments, the particle beamirradiates the surface of the sample at a non-normal angle, i.e., withat least a slight tilt. As a result, the energetic primary beam strikesthe side walls of the contact holes, rather than the bottom. The surfaceof the sample is negatively precharged, so that secondary electronsemitted from the side walls and upper edge of the contact holes aredriven down toward the bottom of the holes. The secondary electrons,however, are substantially less energetic than the electrons in theprimary beam. Therefore, the secondary electrons are less able than theprimary electrons to penetrate through thin residue layers that mayremain at the bottom of the contact holes. As a result, the measurementof specimen current using an angled particle beam can, under someconditions, provide a more sensitive indicator of etch state than can beachieved using a conventional, normal-incidence beam.

The angled beam may be used to enhance the sensitivity of specimencurrent measurements in other applications, as well, as will be apparentto those skilled in the art. For example, the angled beam may be used tomeasure punch-through of contact hole side walls, which leads to currentleakage through the side walls to nearby polysilicon structures. Asanother example, contact holes produced at the periphery of a siliconwafer may be tilted due to the effect of fringing fields in thedielectric etch process. The electron beam may be angled so that theelectrons still strike the bottom of these tilted contact holes or toensure the beam hits the contact hole side wall at a desired angle.

In another embodiment, the sample is irradiated simultaneously by acharged particle beam and by electromagnetic radiation, i.e., by a beamof photons, typically a beam of visible, near-infrared or ultravioletlight. This technique is useful, for example, in assessing the etchquality of contact holes used to contact P-N junctions in functionaldice (or otherwise connected to P-N junctions), which are fabricated ina semiconductor wafer. When such a junction is biased by charge from acharged particle beam alone, little or no specimen current may flowthrough to the semiconductor substrate if the charge-induced voltagereverse-biases the junction (since the junction acts as a non-conductingreverse-biased diode). If the junction is irradiated with light at aphoton energy greater than the semiconductor bandgap energy, however,electron-hole pairs will be created at the P-N junction and in thesubstrate, so that a significant specimen current may flow through theP-N junction. The combination of particle beam and electromagneticirradiation can also be used to measure other aspects of devicesproduced on semiconductor wafers, particularly front-end devicestructures, as will be apparent to those skilled in the art.

As noted above, in measurements of specimen current flowing throughcontact holes, it is frequently advantageous to negatively bias theupper surface of the sample, i.e., the surface on which the electronbeam is incident. In systems known in the art, the negative bias iscreated by operating the electron beam at high energy, in the negativecharging domain (i.e., the energy range in which the total yield ofbackscattered and secondary electrons from the wafer is less than theprimary electron beam current), in order to precharge the surface.High-energy irradiation, however, can cause damage to the sample.Therefore, in some embodiments of the present invention, an electrodenear the surface is used to apply a negative bias potential while thesurface is irradiated by the electron beam. The bias potential causesthe secondary electrons emitted from the surface to return to thesurface, thus creating a net negative precharge, without the need forhigh-intensity, high-energy irradiation of the surface as in systemsknown in the art.

There is therefore provided, in accordance with an embodiment of thepresent invention, a method for process monitoring, including:

receiving a sample having a first layer that is at least partiallyconductive and a second layer formed over the first layer, followingproduction of contact openings in the second layer by an etch process,the contact openings including a plurality of test openings havingdifferent, respective transverse dimensions;

directing a beam of charged particles to irradiate the test openings;

measuring, in response to the beam, at least one of a specimen currentflowing through the first layer and a total yield of electrons emittedfrom a surface of the sample, thus producing an etch indicator signal;and

analyzing the etch indicator signal as a function of the transversedimensions of the test openings so as to assess a characteristic of theetch process.

In an aspect of the invention, analyzing the etch indicator signalincludes assessing a residual thickness of the dielectric layer at abottom of the test openings as a function of the transverse dimensions.In one embodiment, the test openings include a first opening having afirst transverse dimension, and at least a second opening having asecond transverse dimension that is less than the first transversedimension, and the method includes controlling the etch process, inresponse to the etch indicator signal, so that the first opening issufficiently deep to reach the first layer, while at least the secondopening is not sufficiently deep to reach the first layer. In anotherembodiment, the test openings further include a third opening, having athird transverse dimension intermediate the first and second transversedimensions, and analyzing the etch indicator signal includes detecting apotential process defect when the etch indicator signal indicates thatthe third opening is not sufficiently deep to reach the first layer.

In another aspect of the invention, the sample may have a barrier layerformed between the first and second layers, and assessing the residualthickness may include analyzing the etch indicator signal after etchingthe second layer in order to assess an integrity of the barrier layer,and then analyzing the etch indicator signal after etching the barrierlayer, typically in order to verify that at least some of the contactopenings have been etched through the barrier layer to the first layer.

In still another aspect of the invention, analyzing the etch indicatorsignal includes assessing a critical dimension of a bottom of the testopenings as a function of the transverse dimensions.

Optionally, analyzing the etch indicator signal includes measuring abeam current of the beam of charged particles, and analyzing a ratio ofthe etch indicator signal to the beam current. Alternatively, measuringat least one of the specimen current and the total yield includesmeasuring the total yield of the electrons emitted from the surface ofthe sample and further includes measuring a primary current of the beam,and taking a difference between the primary current and the total yieldto determine the etch indicator signal.

In a disclosed embodiment, the plurality of test openings includesmultiple groups of the test openings in respective test areas, which aredistributed in different locations across the sample, and directing thebeam includes positioning at least one of the beam and the sample so asto irradiate each of at least two of the test areas in turn. Analyzingthe etch indicator signal may include evaluating a variation of the etchindicator signal across the sample so as to assess a uniformity of theetch process.

Typically, directing the beam includes operating the beam so as toprecharge a surface of the sample in proximity to the test openings, soas to facilitate measurement of the specimen current.

In an aspect of the invention, the sample includes a semiconductorwafer, and the contact openings include at least one of contact holes,trenches and vias. At least some of the contact openings not included inthe plurality of test openings may belong to multiple microelectroniccircuits on the wafer, wherein the circuits are separated by scribelines, and the test openings are located on one of the scribe lines.

In another aspect of the invention, receiving the sample includesreceiving the sample with a photoresist layer overlying the secondlayer, the photoresist layer having been used in etching the contactopenings, and analyzing the etch indicator signal includes monitoringthe etch indicator signal while irradiating the test area, prior toremoving the photoresist layer. The method may include, if the etchindicator signal indicates that a residual thickness of the second layerat a bottom of one or more of the test openings is greater than apredetermined limit, further etching the second layer using thephotoresist layer so as to increase the depth.

In one embodiment, analyzing the etch indicator signal includesdetecting a residue within the contact openings, and the methodirradiating the sample with the beam of charged particles so as toremove the residue.

Optionally, directing the beam includes directing a pulsed beam of thecharged particles to irradiate the test openings, and measuring at leastone of the specimen current and the total yield of electrons includesmeasuring a time variation of the specimen current by capacitivecoupling to the sample.

There is also provided, in accordance with an embodiment of the presentinvention, a method for process monitoring, including:

receiving a sample having a first layer that is at least partiallyconductive and a second layer formed over the first layer, followingproduction of contact openings in the second layer by an etch process,the contact openings including at least first and second arrays of testopenings, characterized by different, respective first and secondspacings between the test openings in the first and second arrays;

directing a beam of charged particles to irradiate the test openings;

measuring, in response to the beam, at least one of a specimen currentflowing through the first layer and a total yield of electrons emittedfrom a surface of the sample, thus producing an etch indicator signal;and

analyzing the etch indicator signal as a function of the spacings of thearrays of the test openings so as to assess a characteristic of the etchprocess.

In an aspect of the invention, analyzing the etch indicator signalincludes assessing a residual thickness of the dielectric layer at abottom of the test openings as a function of the spacings. Typically,the first spacing is substantially greater than the second spacing, andthe method includes controlling the etch process, in response to theetch indicator signal, so that the test openings in the first array aresufficiently deep to reach the first layer, while the test openings inthe second array are not sufficiently deep to reach the first layer.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for monitoring a process carried out on asample, the method including:

directing a beam of charged particles to irradiate the sample along abeam axis that deviates substantially in angle from a normal to asurface of the sample;

measuring, in response to incidence of the beam on the sample, aspecimen current flowing through the sample; and

analyzing the specimen current so as to assess a characteristic of theprocess.

Typically, the sample has a first layer that is at least partiallyconductive and a second layer formed over the first layer, and theprocess includes an etch process, which is applied to the sample so asto produce contact openings in the second layer, and directing the beamincludes irradiating the contact openings, and analyzing the specimencurrent includes assessing the etch process. Some of the contact holesmay be characterized by a tilt relative to the normal to the surface,and directing the beam may then include angling the beam so as tocompensate for the tilt.

Typically, the contact openings have side walls and a bottom, anddirecting the beam may additionally or alternatively include angling thebeam so that more of the charged particles strike the side walls thanstrike the bottom. In an aspect of the invention, the contact openingsare characterized by an aspect ratio, and directing the beam includesaligning the beam axis at an angle that deviates from the normal to thesurface by at least an arctangent of an inverse of the aspect ratio.

There is further provided, in accordance with an embodiment of thepresent invention, a method for process monitoring, including:

directing a beam of charged particles to irradiate a surface of asample, whereby electrons are emitted from the surface;

applying an electric field in a vicinity of the surface, so as to causeat least a portion of the emitted electrons to return to the surface,thereby generating a negative precharge at the surface; and

receiving a signal produced by the sample in response to the beam andthe negative precharge.

Typically, the sample has a first layer that is at least partiallyconductive and a second layer formed over the first layer, and thenegative precharge is formed on the surface of the dielectric layer.

In an aspect of the invention, directing the beam includes operating thebeam during a precharging interval so as to generate the negativeprecharge at the surface, and then operating the beam after theprecharging interval so as to generate the signal. Typically, operatingthe beam during the precharging interval includes setting the beamsource so that electrons have an energy in a positive charging domain ofthe surface of the sample.

There is moreover provided, in accordance with an embodiment of thepresent invention, a method for testing a semiconductor device,including:

irradiating a junction in the semiconductor device with a first beamincluding electromagnetic radiation;

irradiating the device with a second beam including charged particles,so that at least some of the charged particles are incident on thejunction substantially simultaneously with the electromagneticradiation; and

measuring, in response to incidence of the first and second beams on thejunction, a property of the device.

In an aspect of the invention, measuring the property includes formingan electronic image of the device.

In another aspect of the invention, the junction includes asemiconductor material, and irradiating the junction with the first beamincludes irradiating the junction with photons having an energy greaterthan or equal to a bandgap of the semiconductor material. Typically, thejunction includes a P-N junction.

Additionally or alternatively, measuring the property includes measuringa current flowing through the device, wherein a dielectric layer isformed over the junction, and a contact hole is formed through thedielectric layer in order to contact the junction, and whereinirradiating the junction with the first and second beams includesirradiating an interior of the contact hole, and wherein measuring thecurrent includes assessing a characteristic of the contact hole based onthe current. Typically, assessing the characteristic includes assessinga suitability of the contact hole to make a conductive electricalcontact with the junction.

There is furthermore provided, in accordance with an embodiment of thepresent invention, a method for process monitoring, including:

receiving a sample having a first layer that is at least partiallyconductive and a second layer formed over the first layer, followingproduction of contact openings in the second layer by an etch process;

directing a beam of charged particles to irradiate one or more of thecontact openings;

measuring a primary current of the beam and a total yield of electronsemitted from a surface of the sample in response incidence of the beamon the contact openings; and

analyzing a relation between the primary current and the total yield ofthe electrons so as to assess a characteristic of the etch process.

Analyzing the relation may include analyzing a difference between theprimary current and the total yield or, additionally or alternatively,analyzing a ratio between the primary current and the total yield.

There is also provided, in accordance with an embodiment of the presentinvention, a method for process monitoring of a sample having a firstlayer that is at least partially conductive and a second layer formedover the first layer, wherein contact openings are formed in the secondlayer by an etch process, the method including:

determining, for a given set of characteristics of the contact openings,a threshold level of an etch indicator signal, which is produced bymeasuring at least one of a specimen current flowing through the firstlayer and a total yield of electrons emitted from a surface of thesample in response to irradiation of the contact openings by a beam ofcharged particles;

directing the beam of charged particles to irradiate each of a pluralityof the contact openings that have the given set of characteristics andare disposed at different, respective positions over a surface of thesample;

determining, in response to the beam, the etch indicator signal producedat each of the respective positions of the plurality of the contactopenings; and

comparing the etch indicator signal produced at the respective positionsto the threshold level so as to assess a characteristic of the etchprocess.

Typically, comparing the etch indicator signal includes determining, ifan absolute magnitude of the specimen current falls below the thresholdlevel by more than a predetermined margin, that at least some of thecontact openings are underetched.

Additionally or alternatively, determining the threshold level includesfinding the level of the etch indicator signal that corresponds toetching of the contact openings through the second layer to expose thefirst layer within the opening. In a disclosed embodiment, finding thelevel includes calibrating the threshold level in a procedure performedon a test sample, for subsequent application in assessing thecharacteristic of the etch process performed on other samples.Typically, calibrating the threshold level includes making measurementsof the etch indicator signal generated by the test sample, and comparingthe measurements to at least one of a cross-sectional profile of thecontact openings in the test sample and a conductivity of electricalcontacts made through the contact openings in the test sample.

In an embodiment of the invention, the sample has a barrier layer formedbetween the first and second layers, and finding the level of the etchindicator signal includes finding a first level that corresponds toetching of the contact openings through the second layer to expose thebarrier layer, and finding a second level that corresponds to etching ofthe contact openings through the barrier layer to expose the first layerwithin the openings. Typically, comparing the etch indicator signalincludes analyzing the etch indicator signal after etching the secondlayer in order to assess an integrity of the barrier layer, and thenanalyzing the etch indicator signal after etching the barrier layer inorder to verify that at least some of the contact openings have beenetched through the barrier layer to the first layer.

The method may include evaluating a variation of the etch indicatorsignal across the sample so as to assess a uniformity of the etchprocess, and signaling that a process fault has occurred if thevariation of the etch indicator signal across the sample is greater thana predetermined maximum.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for process monitoring of a sample having afirst layer that is at least partially conductive and a second layerformed over the first layer, wherein contact openings are formed in thesecond layer by an etch process, the method including:

directing a beam of charged particles to irradiate each of a pluralityof the openings that share a given set of characteristics and aredisposed at different, respective positions across the sample;

measuring at least one of a specimen current flowing through the firstlayer and a total yield of electrons emitted from a surface of thesample in response to irradiation of the contact openings by the beam ofcharged particles, thus producing an etch indicator signal as a functionof the respective positions of the plurality of the openings; and

evaluating a variation of the etch indicator signal across the sample soas to assess a uniformity of the etch process.

In an aspect of the invention, evaluating the variation includesdetermining that a process fault has occurred if the variation of theetch indicator signal across the sample is greater than a predeterminedmaximum.

There is further provided, in accordance with an embodiment of thepresent invention, apparatus for etching a sample having a first layerthat is at least partially conductive and a second layer formed over thefirst layer, contact openings having been created in the second layer byan etch process, the contact openings including a plurality of testopenings having different, respective transverse dimensions, theapparatus including:

a test station, which includes:

-   -   a particle beam source, which is adapted to direct a beam of        charged particles to irradiate the test openings; and    -   a current measuring device, which is coupled to measure, in        response to the beam, at least one of a specimen current flowing        through the first layer and a total yield of electrons emitted        from a surface of the sample, thus producing an etch indicator        signal; and

a controller, which is adapted to analyze the etch indicator signal as afunction of the transverse dimensions of the test openings so as toassess a characteristic of the etch process.

There is moreover provided, in accordance with an embodiment of thepresent invention, apparatus for etching a sample having a first layerthat is at least partially conductive and a second layer formed over thefirst layer, contact openings having been created in the second layer byan etch process, the contact openings including at least first andsecond arrays of test openings, characterized by different, respectivefirst and second spacings between the test openings in the first andsecond arrays, the apparatus including:

a test station, which includes:

-   -   a particle beam source, which is adapted to direct a beam of        charged particles to irradiate the test openings; and    -   a current measuring device, which is coupled to measure, in        response to the beam, at least one of a specimen current flowing        through the first layer and a total yield of electrons emitted        from a surface of the sample, thus producing an etch indicator        signal; and

a controller, which is adapted to analyze the etch indicator signal as afunction of the spacings of the arrays of the test openings so as toassess a characteristic of the etch process.

There is furthermore provided, in accordance with an embodiment of thepresent invention, apparatus for monitoring a process carried out on asample, the apparatus including:

a particle beam source, which is adapted to direct a beam of chargedparticles to irradiate the sample along a beam axis that deviatessubstantially in angle from a normal to a surface of the sample;

a current measuring device, which is coupled to measure, in response tothe beam, a specimen current flowing through the sample; and

a controller, which is adapted to analyze the specimen current so as toassess a characteristic of the etch process.

There is also provided, in accordance with an embodiment of the presentinvention, apparatus for process monitoring, including:

a particle beam source, which is adapted to direct a beam of chargedparticles to irradiate a surface of a sample, whereby electrons areemitted from the surface;

a bias electrode, which is adapted to apply an electric field in avicinity of the surface, so as to cause at least a portion of theelectrons emitted during the precharging interval to return to thesurface, thereby generating a negative precharge at the surface; and

a current measuring device, which is coupled to receive a signalproduced by the sample in response to the beam and the negativeprecharge.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for testing a semiconductor device,including:

a radiation source, which is adapted to irradiate a junction in thesemiconductor device with a first beam including electromagneticradiation;

a particle beam source, which is adapted to irradiate the device with asecond beam including charged particles, so that at least some of thecharged particles are incident on the junction substantiallysimultaneously with the electromagnetic radiation; and

a measuring element, which is adapted to measure, in response toincidence of the first and second beams on the junction, a property ofthe device.

There is further provided, in accordance with an embodiment of thepresent invention, apparatus for monitoring an etch process applied to asample having a first layer that is at least partially conductive and asecond layer formed over the first layer, following production ofcontact openings in the second layer by the etch process, the apparatusincluding:

a particle beam source, which is adapted to direct a beam of chargedparticles to irradiate one or more of the contact openings;

a beam current detector, for detecting a primary current of the beam;

a secondary electron detector, for detecting a total yield of electronsemitted from a surface of the sample in response incidence of the beamon the contact openings; and

a controller, which is adapted a relation between the primary currentand the total yield so as to assess a characteristic of the etchprocess.

There is moreover provided, in accordance with an embodiment of thepresent invention, apparatus for monitoring a process applied to asample having a first layer that is at least partially conductive and asecond layer formed over the first layer, contact openings having beencreated in the second layer by an etch process, the apparatus including:

a test station, including:

a particle beam source, which is adapted to direct a beam of chargedparticles to irradiate each of a plurality of the contact openings thatare disposed at different, respective positions over a surface of thesample; and

a current measuring device, which is adapted to produce an etchindicator signal by measuring, in response to irradiation of each of theplurality of the contact openings by the beam of charged particles, atleast one of a specimen current flowing through the first layer and atotal yield of electrons emitted from a surface of the sample; and

a controller, which is adapted to store a calibrated threshold level ofthe etch indicator signal for a given set of properties of the etchprocess, and to compare the respective etch indicator signal producedwith respect to each of the plurality of the contact openings to thethreshold level so as to assess a characteristic of the etch process.

There is furthermore provided, in accordance with an embodiment of thepresent invention, apparatus for monitoring a process applied to asample having a first layer that is at least partially conductive and asecond layer formed over the first layer, contact openings having beenformed in the second layer by an etch process, the apparatus including:

a test station, which includes:

-   -   a particle beam source, which is adapted to direct a beam of        charged particles to irradiate each of a plurality of the        openings that are disposed at different, respective positions        across the sample; and    -   a current measuring device, which is adapted to measure at least        one of a specimen current flowing through the first layer and a        total yield of electrons emitted from a surface of the sample in        response to irradiation of the contact openings by the beam of        charged particles, thus producing an etch indicator signal as a        function of the respective positions of the plurality of the        openings; and

a controller, which is adapted to evaluate a variation of the etchindicator signal across the sample so as to assess a uniformity of theetch process.

There is also provided, in accordance with an embodiment of the presentinvention, a method for process monitoring of a sample having a firstlayer that is at least partially conductive, a second, barrier layerformed over the first layer, and a third, dielectric layer formed overthe second layer, the method including:

etching contact openings in the third layer in a first etch process;

directing a beam of charged particles to irradiate the contact openingsin a first monitoring step following the first etch process;

measuring at least one of a specimen current flowing through the firstlayer and a total yield of electrons emitted from a surface of thesample in response to irradiation of the contact openings by the beam ofcharged particles in the first monitoring step, thus producing a secondetch indicator signal;

evaluating the first etch indicator signal to assess a firstcharacteristic of the first etch process;

further etching the contact openings from the third layer into thesecond layer in a second etch process;

directing the beam of charged particles to irradiate the contactopenings in a second monitoring step following the second etch process;

measuring the at least one of the specimen current flowing through thefirst layer and the total yield of the electrons emitted from thesurface of the sample in response to irradiation of the contact openingsby the beam of charged particles in the second monitoring step, thusproducing a second etch indicator signal; and

evaluating the second etch indicator signal to assess a secondcharacteristic of the second etch process.

Typically, evaluating the first etch indicator signal includes assessingan integrity of the second layer.

Additionally or alternatively, evaluating the second etch indicatorsignal includes verifying that at least some of the contact openingshave been etched through the second layer to the first layer.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for process monitoring of a sample having afirst layer that is at least partially conductive, a second, barrierlayer formed over the first layer, and a third, dielectric layer formedover the second layer, the apparatus including:

an etch station, which is adapted to form contact openings in the thirdlayer in a first etch process, and subsequently to further etch thecontact openings from the third layer into the second layer in a secondetch process;

a test station, which includes:

-   -   a particle beam source, which is adapted to direct a beam of        charged particles to irradiate the contact openings; and    -   a current measuring device, which is adapted to measure at least        one of a specimen current flowing through the first layer and a        total yield of electrons emitted from a surface of the sample in        response to irradiation of the contact openings by the beam of        charged particles, thus producing a first etch indicator signal        following the first etch process and a second etch indicator        signal following the second etch process; and

a controller, which is adapted to evaluate the first etch indicatorsignal in order to assess a first characteristic of the first etchprocess and to evaluate the second etch indicator signal in order toassess a second characteristic of the second etch process.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic top view of a semiconductor wafer with a testpattern comprising an array of contact holes formed therein, inaccordance with an embodiment of the present invention;

FIG. 1B is a schematic top view of another test pattern comprisingsparse and dense arrays of contact holes, in accordance with anembodiment of the present invention;

FIG. 1C is a schematic top view of a further test pattern comprisingarrays of contact holes of different diameters and densities, inaccordance with an embodiment of the present invention;

FIGS. 2A-2E are schematic, sectional illustrations of an area of asemiconductor wafer, showing a contact hole etched into the wafer underdifferent process conditions;

FIG. 3A is a block diagram that schematically illustrates apparatus fortesting contact hole production, in accordance with an embodiment of thepresent invention;

FIG. 3B is a schematic, sectional, detail view of a semiconductor waferunder test, illustrating periodic measurement of specimen current, inaccordance with an embodiment of the present invention;

FIG. 3C is a schematic plot showing waveforms of an AC electron beamirradiating a semiconductor wafer and specimen current measured as aresult of the irradiation, in accordance with an embodiment of thepresent invention;

FIG. 4 is a schematic, sectional view of the array of contact holes inthe test pattern of FIG. 1, taken along a line IV—IV;

FIG. 5 is a schematic plot of specimen current as a function of holesize, for the array of contact holes shown in FIG. 4;

FIGS. 6-8 are schematic plots of specimen current measured as a functionof contact hole position over the surface of a sample, in accordancewith an embodiment of the present invention;

FIGS. 9A and 9B are schematic plots of specimen current measured as afunction of contact hole position over the surface of a sample,illustrating calibration thresholds used in contact hole monitoring, inaccordance with an embodiment of the present invention;

FIG. 10 is a schematic top view of a cluster tool that includes acontact hole test station, in accordance with an embodiment of thepresent invention;

FIG. 11 is a schematic, sectional view of a contact hole on which anelectron beam is incident at a non-normal angle, in accordance with anembodiment of the present invention;

FIG. 12 is a schematic, pictorial illustration of a biasing electrodeused in conjunction with an electron beam to precharge a surface of asample, in accordance with an embodiment of the present invention; and

FIG. 13 is a schematic, sectional illustration showing simultaneousirradiation of a sample by electron and light beams, and measurement ofthe resultant specimen current, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS System and Method Overview

Reference is now made to FIG. 1A, which is a schematic top view of asemiconductor wafer 20 with a test pattern 22 formed thereon, inaccordance with an embodiment of the present invention. The testpattern, comprising an array of contact holes 26, is shown enlarged inan inset. Although only the single test pattern 22 is shown in FIG. 1,multiple test patterns may be distributed over the surface of wafer 20.Other types of test openings and test patterns may also be used, such asthe types described in the above-mentioned U.S. patent application Ser.No. 10/209,087. The test patterns may be located on scribe lines 24between adjacent dice on wafer 20, so as to minimize the loss of usefulspace on the wafer. Additionally or alternatively, the methods ofcontact hole evaluation described hereinbelow may be applied, mutatismutandis, to contact openings formed in functional areas of the dice.

Holes 26 in test pattern 22 may be graduated in diameter from large tosmall, as shown in the figure. The size gradation of the holes isparticularly useful in assessing the state of an etch process used inprocessing wafer 20, as described below with reference to FIGS. 4 and 5.Typically, the holes are designed to range between 50 nm and 1 μm indiameter and are spaced at least one diameter apart. These dimensionsand spacing of the holes and of the test pattern are cited by way ofexample, however, and other dimensions and spacing may likewise be used(The spacing between holes 26 may also be varied, as described below.)Although FIG. 1A shows a single row of holes 26, the holes may also bearranged in a two-dimensional pattern, and some of the holes may havethe same diameter. The test pattern may also include other types ofcontact openings (not shown), such as trenches or vias.

FIG. 1B is a schematic top view of another test pattern 21, which may beformed on a semiconductor wafer, in accordance with an embodiment of thepresent invention. Pattern 21 comprises contact holes 26 arrayed in twopatterns: a sparse pattern 23, and a dense pattern 25. Typically, holes26 have the same diameter in both the sparse and dense patterns. Asnoted above, the etch rate of contact holes is typically lower in densepattern 25 than in sparse pattern 23, due to micro-loading effects. Thespacing of the contact holes in dense and sparse arrays in the teststructure is typically chosen to represent limiting cases of actualcontact hole spacing for in-die patterns. Therefore, by measuringspecimen current through contact holes in the dense and sparse arrays,it is possible to detect etch problems that may occur within in-diepatterns due to micro-loading.

FIG. 1C is a schematic top view of still another test pattern 36 withinthe area of a scribe line 24 on a semiconductor wafer, in accordancewith an embodiment of the present invention. Pattern 36 combines theprinciples of the patterns shown above in FIGS. 1A and 1B. Pattern 36comprises arrays 23 and 25 of sparse and dense contact holes, whosediameters are approximately equal to the critical dimension (CD) of thein-die functional contact holes that are produced on the wafer. Inaddition, the pattern comprises a dense array 27 and a sparse array 29of contact holes having a smaller diameter than the in-die contactholes; and a dense array 31 and a sparse array 33 of contact holeshaving a larger diameter than the in-die contact holes. An alignmenttarget 35 is typically provided in pattern 36 to facilitate opticalalignment of an inspection system that is used to make specimen currentmeasurements on the pattern, as described hereinbelow. Pattern 36 mayalso include an area that includes no contact holes, for use inestablishing a calibration baseline for the specimen currentmeasurements made on the pattern.

FIGS. 2A-2E are schematic, sectional illustrations of an area of asemiconductor wafer, showing formation of contact hole 26 underdifferent process conditions. In a typical application, a non-conductingoxide layer 30 is formed over a silicon substrate layer 28, andphotoresist (not shown in the figure) is deposited on the oxide layer.After photolithographic exposure of the photoresist to define thelocations and dimensions of contact openings in the oxide layer, anetching process is applied to create the contact holes.

In the exemplary application shown in these figures, hole 26 is meant toprovide a contact to a region 34 of substrate layer 28 that containsTiSi₂ for enhanced conductivity. Region 34 may be part of a transistorstructure, formed within layer 28 by methods known in the art. Oxidelayer 30 typically comprises materials such as undoped silicon glass(USG), phosphorus silicon glass (PSG), boron phosphorus silicon glass(BPSG), carbon-doped oxide (CDO) or low-k dielectrics. A barrier layer(not shown in this figure), sometimes referred to as an etch stop layer,which is typically made of silicon nitride, silicon carbide or a low-Kbarrier material, such as Applied Materials BLOk™, may be added betweenthe silicon substrate and the dielectric. The structure illustrate inthese figures, however, is shown solely by way of example, and holes 26may likewise be made in and adjacent to other structures. Similarly,such contact holes may be used to contact intermediate semiconductor orconductive layers (not shown) formed above substrate 28, rather thancontacting the substrate itself directly.

Holes 26 in test pattern 22 are formed by the same processes of materialdeposition, photolithography and etching as are the functional circuitfeatures on the wafer that the pattern is intended to test. Within holes26, substrate layer 28 is exposed to the same extent as it is exposed byetching of contact holes of similar diameter and spacing in functionalareas of the wafer. A measurement of the specimen current generated whenpattern 22 is irradiated by an electron beam is indicative of the extentto which layer 28 (or an overlying semiconductor or conductive layer) isexposed within the holes. To facilitate this measurement, a conductivecontact pad (not shown in the figure) may be formed on the underside ofwafer 20, below pattern 22. Apparatus and methods used in measuring thespecimen current are shown in the figures that follow and are describedwith reference thereto.

FIG. 2A shows a perfectly-etched, open hole, i.e., a contact hole thatcleanly exposes layer 28 as desired. The remaining figures in this setshow the results of different process problems or defects. In FIG. 2B,hole 26 is underetched, typically due to a problem in the etchingprocess or in the uniformity of oxide layer 30, for example.Consequently, the area of layer 28 that is exposed within hole 26 issmaller than it should be. In this case, the specimen current generatedwhen the area of hole 26 is irradiated by an electron beam will besmaller than the current generated in the case of FIG. 2A. When the holeis filled with metal or other conductive material in order to contactlayer 28, the contact resistance may be higher than it should.

In FIG. 2C, the etching process is too strong or has continued for toomuch time, leading to overetching of hole 26. In this case, the specimencurrent will typically be greater than in the case of FIG. 2A.Overetching may have a deleterious effect on region 34 and on otherstructures, and may also lead to deposit of contaminants at the bottomof hole 26.

FIG. 2D shows a case of severe underetching, in which hole 26 stopsshort of reaching layer 28, typically due to some serious processdefect. For this sort of closed contact hole, the measured specimencurrent will be very low, and the contact resistance when the hole isfilled with metal will be very high.

Finally, in FIG. 2E, although hole 26 was properly etched, a contaminant38, such as photoresist residue or fluorocarbon polymer, is deposited atthe bottom of the hole. This contaminant will typically cause a decreasein the measured specimen current. If the residue is not removed, it maycause a high contact resistance when hole 26 is filled with metal. Thishigh contact resistance is a critical process problem, which cangenerally be detected (using methods known in the art) only many processsteps later, after the metal layer has been deposited in the holesusually by electrical testing.

FIG. 3A is a block diagram that schematically illustrates a station 40for contact hole inspection, in accordance with an embodiment of thepresent invention. Station 40 comprises a chamber 42, containing amotion stage 44 on which wafer 20 is placed during inspection. Anelectron gun 46 (or other charged particle source) directs a beam atwafer 20, while an ammeter 48 measures the specimen current generated inthe wafer. The ammeter is typically electrically coupled to the lowerside of wafer 20, in electrical contact with substrate layer 28.Alternatively, the ammeter may be coupled directly to an intermediatesemiconductor or conductive layer in the wafer, assuming that the layerson wafer 20 are suitably configured to enable such coupling. As notedabove, the wafer may include one or more contact pads for use incoupling ammeter 48 to the substrate or intermediate layer.

The electron beam generated by gun 46 typically has a diameter andenergy parameters that can be controlled as required by the application.The diameter may be adjusted to cover a single contact hole on thewafer, or expanded to irradiate several holes at once or to prechargethe wafer surface. An adjustment range of 0.5-30 μm in beam diameter isgenerally adequate for these purposes. The electron energy of the gunmay be variable, typically between about 100 and 5000 eV, so as to coverboth positive and negative charging domains of the materials in wafer20. (The positive charging domain is the range of electron energies inwhich the total yield of secondary and backscattered electrons from thesurface layer is greater than the primary electron beam current, whilethe negative charging domain in the range in which the total yield isless than the primary beam current. These phenomena, which are wellknown in the art, are described in the above-mentioned book by Yacobi etal. on pages 38-39.) A suitable electron gun for this purpose, forexample, is the EKF 1000 small-spot electron source, produced by OmicronNanoTechnology GmbH (Taunusstein, Germany). This gun is considerablysmaller and less expensive than the high-resolution electron beamdevices used in typical SEM systems. Alternatively, electron guns ofother types, as well as other types of particle beams, may be used instation 40.

The specimen current due to irradiation of contact holes in wafer 20 istypically measured in steady state. For this purpose, the area of thecontact hole to be irradiated is precharged by the beam from gun 46.This precharging may take place as a separate, preliminary stage, beforemaking the specimen current measurements, or it may alternatively becarried out simultaneously with the measurements. The wafer surface maybe negatively precharged, by operating the electron gun at an energy inthe negative charging domain. For photoresist, this condition typicallyholds for all values of the electron beam energy. For SiO₂, a higherbeam energy, preferably above 2 keV, can be used to give negativecharging. Alternatively, a very low-energy beam can be used for negativecharging.

Further alternatively, a bias electrode 53, which is negatively biasedby a biasing power supply 55, may be used to induce negative charging bylow-energy electrons. This application of the bias electrode isdescribed below in detail with reference to FIG. 12. As yet a furtheralternative, negative charging of the surface may be achieved byapplying an appropriate electric field bias to the wafer surface, usinga charge control plate as described in U.S. patent application Ser. No.08/892,734, which is assigned to the assignee of the present patentapplication, and whose disclosure is incorporated herein by reference.In any case, negative precharging of the wafer surface causes holes 26to act as Faraday cups, so that relatively few electrons escape from theholes.

Stage 44 positions wafer 20 so that each of contact holes 26 to betested is properly located in turn in the beam of gun 46. Given theminimum diameter of the electron beam, positioning resolution of about±3 μm is generally sufficient unless specific, individual contact holesare to be measured. For simplicity and economy of space, stage 44 maycomprise an R-theta (translation/rotation) stage. Alternatively oradditionally, any other type of motion system with sufficient accuracymay be used for this purpose. For example, the stage may provide X-Ytranslation, or gun 46 may be translated over wafer 20, or the electronbeam itself may be deflected. When test holes or test patterns areprovided on wafer 20 at multiple locations, stage 44 may position thewafer (or the electron gun may be translated or its beam deflected) sothat several of these test holes or patterns are irradiated by theelectron beam in succession. The specimen current is measured at eachhole location, in order to ensure that contact hole uniformity ismaintained over the entire wafer, as described further hereinbelow.Additionally or alternatively, if different test holes or test patternson the wafer are designed to test different sizes or shapes of contactopenings, the specimen current can be measured for each hole size orpattern type.

During the specimen current measurements, the beam energy of gun 46 istypically set to be in the negative charging domain of the topdielectric (background) layer, in order to provide optimal contrastbetween good, open contact holes and those that are closed orunderetched. (As noted above, “open” contact holes are those that whenfilled with conductive material will be electrically conductive with lowresistance; while holes that are closed, underetched or have residueremaining at the bottom may be electrically unconnected or exhibit highresistance when filled with the conductive material.) Typically, lowerbeam energy enhances the sensitivity of the measurement to thin layersof residual dielectric material at the bottom of the contact holes.Optionally, a number of different electron beam energies may be used totest the specimen current at a number of different points on the yieldcurve.

FIGS. 3B and 3C illustrate an alternative method for measuring specimencurrent, in accordance with another embodiment of the present invention.If it is not possible to make a good ohmic contact between thesemiconductor or conducting layer at the bottom of the contact holes andammeter 48, the electron beam may be pulsed, and the specimen currentmeasured by capacitive coupling. This arrangement is illustrated in FIG.3B, which is a schematic, sectional view showing a detail of a wafercomprising substrate 28 and dielectric 30, with an additional back-sidedielectric layer 63 below the semiconductor substrate. A photoresistlayer 72 overlying dielectric layer 30 is used in creating contact holes26. An electron beam 61 irradiates the area of contact holes 26. A beamblanking assembly 59 periodically applies a voltage V_(BB) in order topulse the electron beam on and off. The beam blanking assembly maycomprise, for example, a pair of parallel metal plates, between whichthe electron beam travels before passing through an aperture. When avoltage is applied between the plates, the electron beam is deflectedand does not pass through the aperture to reach the wafer. The resultantAC specimen current is measured using an AC ammeter 65, which iscapacitively coupled to substrate 28 through dielectric layer 63.

FIG. 3C schematically shows the time variation of the electron beamcurrent, which is tracked by the time variation of the specimen current.(Possible smoothing and phase shift of the specimen current waveform dueto impedance effects are neglected here for the sake of simplicity.) Anupper specimen current curve 67 illustrates the expected specimencurrent level when contact holes 26 are adequately etched. A lowerspecimen current curve 69 illustrates the reduction in specimen currentthat occurs due to etch problems, such as the underetching shown in FIG.3B.

Alternatively, when a wafer under test has a back-side dielectric layer,the wafer may be mounted on a chuck with pins that contact the back sideof the wafer. A pulsed voltage is applied to the pins in order tolocally penetrate the dielectric layer and establish a good contact withsubstrate 28. The resistance between the pins may be measured in orderto determine when sufficient penetration of the dielectric layer hasbeen achieved.

Returning now to FIG. 3A, the current of electrons emitted from wafer 20may be measured, additionally or alternatively, using a secondaryelectron detector 49, as is known in the art. As noted above, thespecimen current generated in a sample due to irradiation by an electronbeam is equal to the difference between the primary beam current and thetotal electron yield of the specimen due to secondary and backscatteredelectrons. Therefore, it is possible to determine the specimen currentby measuring precisely the primary beam current and the total current ofsecondary and backscattered electrons, without measuring the specimencurrent itself directly. This approach typically requires collection ofsecondary and backscattered electrons high withefficiency—preferably >90%. This high efficiency can be achieved, forexample, using a magnetic immersion lens, which forms at magnetic bottleat the surface of the specimen. Lenses of this sort are described inU.S. Pat. Nos. 4,864,228 and 4,912,405, whose disclosures areincorporated herein by reference. Alternatively, the secondary electroncurrent may be used in conjunction with direct measurement of thespecimen current in order to provide additional information that iscomplementary to the specimen current measurement.

The positioning and operation of gun 46 and stage 44 are controlled by amain controller 50, via a gun control unit 52 and a stage control unit54. Typically, a pre-alignment unit based on a low-resolution opticalmicroscope (OM/PAL) 56 is used by controller 50, via an OM/PAL controlunit 58, to locate the test pattern on the wafer for positioning andalignment purposes. Suitable microscopes for this purpose are made, forexample, by Optem (Fairport, N.Y.). During operation, a vacuum ismaintained in chamber 42 by a vacuum pump 60, which is also controlledand monitored by controller 50, via a vacuum control unit 62. A robot 64inserts wafers into chamber 42 and removes them from the chamber.Controller 50 communicates with the robot via a robot control unit 66.Robot 64 may be used to transfer wafers to and from other stations in acluster tool, as shown below in FIG. 10.

After positioning stage 44 and firing gun 46 to irradiate one or more ofcontact holes 26, controller 50 receives the specimen current measuredby ammeter 48. It compares the measured current to benchmarks that havebeen established for the expected hole size, materials, etch conditionsand other applicable process parameters. Methods for determining thesebenchmarks are described hereinbelow with reference to the figures thatfollow. If the controller determines that the measured current isoutside a predetermined tolerance range of a given benchmark, ittypically interrupts the production process and notifies a systemoperator via a user workstation 68. The operator evaluates the testresults and then implements whatever corrective action may be necessary.

The corrective action may include performing further etching, if thecontact holes are underetched (as shown in FIG. 2B or 2D), or removingpolymer residue that may have been deposited at the bottoms of the holes(FIG. 2E). In the latter case, it may be possible to remove the polymerfilm by high-density electron beam exposure, using electron gun 46. Forthis purpose, electron beam energy between about 5 and 20 keV, with beamcurrent greater than 1 nA, is expected to give satisfactory results.Thus, station 40 may be used for process correction, as well faultdetection.

“Early Warning” Test Pattern

FIG. 4 is a schematic, sectional view of test pattern 22 (FIG. 1), inaccordance with an embodiment of the present invention. The test patternis shown here following completion of an etching process. As notedabove, the diameters of contact holes 26 in pattern 22 are graduatedfrom largest (at the left of the figure) to smallest (at the right),ranged above and below the diameter of a nominal hole 70. The diametersof holes 26 are defined by a photolithographic process applied tophotoresist layer 72, wherein nominal hole 70 is chosen to haveapproximately the same diameter as functional contact holes etched infunctional areas of wafer 20.

The rate at which the etch process creates a contact hole increases asthe contact hole diameter increases. Therefore, the etch state ofnominal hole 70 should be approximately the same as that of functionalcontact holes in wafer 20. As shown in FIG. 4, upon satisfactorycompletion of the etch process, the holes of nominal diameter (i.e., thediameter of hole 70) and larger are etched through completely tosubstrate 28. Below the nominal diameter, the etching rate is slower,and therefore the depth of the holes decreases with decreasing holediameter.

The situation shown in FIG. 4 is indicative of a properly-adjusted etchprocess, in that nominal hole 70 is fully etched through to thesubstrate, without overetching. There is a safety margin in the process(known as a “process window”), in that the contact holes in pattern 22that are slightly narrower than the nominal hole are still etchedthrough to the substrate (so that the nominal hole may be slightlyoveretched, but not to any deleterious extent). If still narrower holeswere etched through to the substrate, there would be a danger ofoveretching the functional holes to which nominal hole 70 corresponds.On the other hand, if the holes just slightly narrower than nominal hole70 were underetched (even if hole 70 still appears to be fully etched),there would be a danger of underetching the functional holes. Thus,monitoring the etching of test pattern 22 can provide an early warningof process defects, so that prompt corrective action can be taken. Ifthese incipient defects were allowed to persist, they could result inimproper etching of functional contact holes in the wafer under test orin other wafers processed subsequently in the same etching chamber asthe current wafer.

FIG. 5 is a schematic plot of specimen current as a function of theinverse of the hole diameter, measured with respect to test pattern 22under two slightly different sets of etch conditions. Each data point inthe plot corresponds to a measurement of specimen current made whileirradiating one of holes 26 with an electron beam. (Typically, the spotsize of the beam is larger than the hole diameter.) An upper curve 80shows the specimen current measured for the set of hole depths shown inFIG. 4. The specimen current decreases gradually in proportion to thehole diameter down to a shoulder value, below which the current dropsmore sharply. This shoulder corresponds to the point at which the holesare no longer fully etched, leaving a highly-resistive dielectric layerat the bottom of the hole, with thickness increasing as hole diameterdecreases. In curve 80, the shoulder occurs several points to the rightof nominal hole 70, indicating that the etch process parameters areproperly adjusted.

A lower curve 82 shows a change in the measured specimen current thatmay occur when the etch parameters drift from proper adjustment. Theshoulder in curve 82 occurs closer to the point of nominal hole 70,although the specimen current measured through the nominal hole stillindicates complete etching. In such a case, controller 50 may alert workstation 68 (FIG. 3) that a process deviation may be occurring, eventhough the etch state of the contact holes in the current wafer is stillsatisfactory. The operator can then correct the etch process before thedeviation becomes severe enough to cause a reduction in the productionyield.

Specimen Current Measurements and Threshold Calibration

FIG. 6 is a plot of specimen current measured as a function of positionacross two test wafers using, for example, station 40 (FIG. 3), inaccordance with an embodiment of the present invention. The verticalaxis, representing the measured current in this figure and in FIGS. 7and 8, is logically reversed, i.e., the measured current is negative,and the magnitude of the current increases from the top to the bottom ofthe plot. The measurements were made on contact holes at differentlocations along a diameter of the wafer, from one side of the wafer tothe other. The contact holes were etched down to silicon substrate 28,without intervention of a nitride stop layer below oxide 30. The etchstates of the holes were verified after measurement by cross-sectionalimaging of the contact holes.

A first curve 90 was measured on a properly-etched wafer, in which thecontact holes were etched for approximately 30% longer than nominal.Based on these measurements, it is possible to define a threshold 92,corresponding to satisfactory, normal etching of the wafer. Thevariation of the specimen current over the surface of the wafer may beused to define control limits, over which the specimen current ispermitted to vary and still be considered within the acceptable range.

A second curve 94 was measured on a wafer of the same type as curve 90,which was etched using non-uniform process parameters. As a consequence,the contact holes in the central area of the wafer were underetched,resulting in low measured specimen current, while those on the peripheryof the wafer were etched properly. Both these conditions are detected bystation 40. A non-uniform profile, such as that of curve 94, istypically sufficient to indicate that a process problem exists and tonotify a system operator or automatically stop processing wafers, evenif the results are not above a specific absolute threshold.

The specimen current measurements shown in FIG. 6, as well as thoseshown in the figures that follow, were compared to cross-sectionalimages of the wafers that were tested, and a good correlation was foundbetween the specimen current levels and the actual etch states of thecontact holes.

FIG. 7 is a plot of specimen current measured as a function of positionof contact holes distributed across another test wafer, in accordancewith an embodiment of the present invention. In this case, the waferincluded a nitride stop layer below the oxide that was etched. The lowspecimen current measured upon irradiation of the central points in thecurve is indicative of underetching of the holes at these points. Towardthe edges of the wafer, the holes were fully etched, down to the nitridelayer. Because of the relatively high conductivity of silicon nitride,relative to silicon (which may have been enhanced by the electron beamirradiation), the specimen current flowing through these holes isconsiderably greater than that shown in FIG. 6.

FIG. 8 is a plot of specimen current measured as a function of positionof contact holes distributed across yet another test wafer, inaccordance with an embodiment of the present invention. This figureillustrates the capability of station 40 to detect residues in etchingof the nitride etch stop layer (which is typically performed as aseparate process step, to remove the barrier layer from the bottom ofcontact holes, after first etching the hole through the overlyingoxide). The nitride layer was etched out of the contact holes on oneside of the wafer, shown to the left in the plot of FIG. 8, but was leftintact on the other side. It can be seen from this figure that themethods of the present invention may be used to monitor not only thestate of an oxide etching process, but also other etching processes,including nitride etching.

FIGS. 7 and 8 thus demonstrate that the methods of the present inventionmay be used to monitor etching of dual dielectric layers (upperdielectric with stop layer below). Etch stop layers are now used in manyapplications, particularly high aspect ratio contact and via processesin devices such as DRAM. The dual dielectric layers are typically etchedin two different, successive etch steps, one for each layer. It isimportant that the first etch step, illustrated by FIG. 7, reaches butdoes not punch through the stop layer. Punch-through may occur, forexample, due to low selectivity in the first etch step or to use of avery thin stop layer. The punch-through would be evidenced by anabnormally large value of the specimen current upon conclusion of thefirst etch step, while underetching in the first etch step gives lowspecimen current, as shown in FIG. 7. The second etch step, in which thecontact holes are etched through the stop layer may be monitored insimilar fashion, but with different threshold levels to indicate properetching.

FIG. 9A shows schematic plots 103, 104, 105 and 106 of specimen current(absolute values), measured as a function of contact hole position for anumber of different samples. The measured currents are used incalibrating absolute process control limits, in accordance with anembodiment of the present invention. Plots 103-106 are measured usingtest structures and measurement methods such as those described above.The measurements of specimen current from specific contact holes arecompared to cross-sectional images of the same contact holes. Some orall of the dies used in the specimen current measurements may besectioned for this purpose. These measurements are then used inestablishing an upper excursion limit 100 and a lower excursion limit102, marking the bounds of measured specimen current values 108 thatcorrespond to acceptable contact holes.

Plot 106 illustrates specimen current measurement values 109 that weremade on an underetched wafer. In this case, only the contact holes onthe wafer periphery were adequately etched. (Whether the peripheralholes are etched differently from those nearer the center of the waferdepends on factors such as the etcher type, process recipe andmaterials, inter alia.)

Plot 105 was taken from a slightly overetched wafer, and demonstratesproperly-etched contact holes over the entire wafer diameter. Properetching of the contact holes is verified by cross-sectional imaging.Plot 105 can be used to establish lower excursion limit 102, based onthe minimal absolute value of the specimen current on this plot, takingthe estimated measurement error (illustrated by the error bars in thefigure) into consideration. If the cross-sectional images show any ofthe holes on this wafer to be underetched, on the other hand, anotherwafer may be etched, using a longer etch time, and may be tested in likemanner to establish the lower excursion limit.

Plot 104 illustrates specimen current measurements made on a waferetched according to an optimized method, in accordance with anembodiment of the present invention, which typically corresponds toextending the etching time by 10-30% compared to that used in generatingplot 105. The minimal specimen current measurement on plot 106 is usedin determining a lower control limit (LCL) 101, again taking intoaccount the estimated measurement error. When the calibration boundsdetermined by the present method are used in monitoring productionwafers, and the measured specimen current drops below LCL 101, an earlywarning signal may be issued to warn of possible process drift.Typically, the specimen current measurements from a number of wafers maybe analyzed statistically in order to set LCL 101, so as to account fornormal etch process variations.

Plot 103 shows specimen current measurements taken from astrongly-overetched wafer. In this case, measurement values 107 areindicative of punch-through of a stop layer below the main dielectriclayer being etched, as described above. The punch-through is verified bycross-sectional images. Upper excursion limit 100 is set to correspondto the maximal specimen current value below the error bounds ofmeasurements 107.

FIG. 9B schematically illustrates specimen current measurements used incalibrating relative control limits, in accordance with an embodiment ofthe present invention. The inventors have found that underetched waferstend to exhibit very high non-uniformity of specimen currentmeasurements taken across the wafer diameter. As shown in FIG. 9B, thenon-uniformity in a plot 112 taken on an underetched wafer may reach100%, as indicated by a maximum current value 113 and a minimum currentvalue 114 reached by this plot. By contrast, plots 110 and 111 show thatfor properly-etched wafers, the non-uniformity is typically no more than10-15%. Non-uniformity is also significant in overetched wafers in whichpunch-through has occurred.

The relative control limit for non-uniformity is thus a single value,indicating the maximal permitted variation among specimen currentmeasurements taken over the diameter of a wafer. It can be determinedfrom plot 111, for example, which shows the measurements made on aslightly-overetched wafer, as verified by cross-sectional imaging. Therelative control limit is typically applied in subsequent measurementson production wafers as an average non-uniformity value (including errorbars). Excursions of the average non-uniformity above the relativecontrol limit are considered to indicate underetching or punch-throughin the contact holes on the wafer under test. The use of such a relativecontrol limit is advantageous in that it provides fast, reliable processmonitoring, which is insensitive to variations in the specimen currentdue to drift in the primary electron beam current.

Alternatively or additionally, the primary electron beam current may bemonitored, and the ratio of the specimen current to the primary beamcurrent may be used as an etch quality indicator.

FIG. 10 is a schematic top view of an etch process cluster tool 120 inwhich test station 40 is integrated, in accordance with an embodiment ofthe present invention. This integration is made possible by the smallsize and simplicity of the components of station 40. Robot 64 receiveswafer 20 through a load lock 21, after photoresist has been depositedover oxide layer 30 and has then been exposed by photolithography toform circuit features including contacts and/or vias, with a suitabletest pattern, such as pattern 22. Since the interior of tool 120 isevacuated, robot 64 is able to transfer wafer 20 from chamber to chamberwithout exposing the wafer to ambient air. Typically, the wafer isinserted in an etching station 124. At this stage, holes 26 are formedthrough layer 30, preferably by a reactive ion etching process. Theforegoing steps are known in the art and are described here solely byway of illustration. Other arrangements of the stations in tool 120 maysimilarly be used.

After etching of holes 26 in wafer 20, the wafer is passed to teststation 40. At this point, the wafer (except for the etched holes) isstill covered by a layer of exposed photoresist. In station 40, thespecimen current from wafer 20 is measured at one or more pre-definedpoints, either in product dies or on test structures or both. Theresults are evaluated, as described above, by a controller 128 (whichmay incorporate the functions of controller 50, shown in FIG. 3).Typically, the controller evaluates the specimen current for multipleholes distributed across the wafer, as shown in the preceding figures,and compares the measured values to both absolute and relativethresholds for the process in question. If the specimen current for allholes measured is within the tolerance range defined by the thresholds,the contact holes in the wafer are deemed to be acceptable. Robot 64then moves wafer 20 into a plasma ashing station 126 for removal of theremaining photoresist, and to a cleaning station 122. If desired, thecontact hole test in chamber 40 may be repeated after the ashing stage.

On the other hand, if the specimen current measured in station 40 is toolow, indicating that the holes have been underetched, robot 64 may beinstructed (automatically or manually) to return the wafer to etchingstation 74 for further etching, to be followed by re-test in station 40.Under these circumstances, controller 128 typically issues an alarm toworkstation 68, as well, indicating to the operator that an adjustmentof process parameters may be needed. Alternatively, controller 128 mayautonomously adjust certain process parameters (increasing or decreasingthe etch duration in etching station 124, for example), in response todeviations of the specimen current from ideal behavior.

Contact Hole Measurements Using an Angled E-Beam

FIG. 11 is a schematic, sectional illustration showing angledirradiation of contact hole 26 by an electron beam 130, in accordancewith an embodiment of the present invention. The tilt angle of beam 130is preferably chosen so that a majority of primary beam electrons do notstrike the bottom of the contact hole. This condition can be achievedwhen the following geometrical condition is satisfied:α>arctan(1/AR),wherein α is the tilt angle, and AR is the aspect ratio (ratio of depthto diameter) of the contact hole.

As a result of the tilt angle, the primary electrons hit the side wallof the hole 26 rather than the bottom. The electron bombardment causesemission of secondary electrons with low energy (typically <50 eV). Thelow-energy secondary electrons can be forced down to the hole bottom,rather than moving out of the hole, by negatively precharging a surface132 of the wafer around the hole. If the contact hole is etched properly(with no residue left at the bottom), the low-energy electron flow willpass through substrate 28 and will thus be measured as a specimencurrent by ammeter 48. If a thin residue (even tens of Angstroms thick),such as under etched dielectric or contaminant 38, is left after theetch, the specimen current will be much lower, due to low penetrationdepth of the low-energy secondary electrons.

Similar results may be achieved using a very low energy (preferably50-500 eV) primary electron beam at normal incidence. The lower energyin either case reduces the interaction volume at the surface of thebottom of the hole and thus substantially increases the sensitivity ofthe specimen current to thin layers.

By contrast, when electron beam 130 operates at higher energy and is notangled, the energetic primary electrons reach the bottom of hole 26. Inthis case, the interaction volume is larger, and the electrons thus passeasily through contaminant 38. Therefore, holes with particularly thinresidues cannot be readily distinguished from holes that have beenetched satisfactorily. Angling electron beam 130 thus provides anelectron energy transformation inside hole 26, which substantiallyenhances sensitivity of the specimen current measurement to very thinresidues at the hole bottom. This method is particularly useful indetecting fluorocarbon polymer residue, organic photoresist residue, andextremely thin oxide, nitride or other dielectric residue (includinglow-k dielectric with a corresponding stop layer). It can be performedby station 40 in-line, providing closed-loop monitoring of reactive ion(plasma) etching, photoresist ashing, and wet polymer cleaning steps.

Tilting of electron beam 130 may be achieved by either mechanical meansor by the use of electron optics to control the beam, or by acombination of both techniques. Some CD SEM systems, such as the AppliedMaterials NanoSEM3D, provide this sort of beam tilt capability.

Negative Precharging Using a Bias Electrode

As noted above, it is desirable in specimen current measurements forcontact hole monitoring to negatively precharge surface 132 of wafer 20.In order to induce a negative precharge on oxide layer 30 using primaryelectron beam 130, it is typically necessary to work at a beam energy ofseveral keV (up to 5 keV depending on the dielectric type). Such highenergy electrons, however, can damage silicide layers and gate oxides inthe wafer, which may lead to semiconductor device degradation, failureor yield loss.

FIG. 12 is a schematic, pictorial illustration, showing how biaselectrode 53 may be used to alleviate this problem, in accordance withan embodiment of the present invention. In this figure, electrode isshown as a ring, with an aperture for beam 130. Alternatively, electrode53 may comprise a fine grid, for example, or may be produced in otherforms, as will be apparent to those skilled in the art. The use of abiased filter mesh of this sort—albeit for other purposes—is describedin European Patent Application EP 0 892 275 A2, whose disclosure isincorporated herein by reference. It is even possible to adapt thebottom electrode (i.e., the electrode next to wafer 20) of a SEMimmersion lens to serve this purpose.

Electrode 53 is negatively biased by power supply 55. The negative biasrepels secondary electrons 134 that are emitted from surface 132 due toincidence of beam 130, without substantially influencing the primaryelectron energy of beam 130. By repelling the low-energy secondaryelectrons back to surface 132, electrode 53 creating a negative netcharge on the dielectric surface. In other words, electrode 53 causesthe total electron charge leaving the surface to be less than the chargeacquired by the surface due to the primary electron irradiation. Theinventors have found that using a 1 keV primary electron beam with a −50V bias on electrode 53 provides satisfactory precharging of surface 132.Electrode 53 was about 1 cm in diameter and was placed about 1.5 mm fromsurface 132. The electrode had a central aperture about 1 mm in diameterthrough which beam 130 passed. This method of precharging may be usedadvantageously in conjunction with the angled beam irradiation methodshown in FIG. 11.

Combined Optical and E-Beam Excitation

FIG. 13 schematically illustrates an active transistor structure 140,which is irradiated simultaneously by e-beam 130 and by an optical beam162 produced by a light source 160, in accordance with anotherembodiment of the present invention. This embodiment is directedparticularly to etch quality assessment of functional contact holes,used in producing microelectronic devices on wafer 20.

Structure 140 is a typical CMOS structure, which includes an NMOStransistor 142 and a PMOS transistor 144. As is known in the art,transistor 142 is situated in a P-well 146, containing N-typesource/drain 148 and a gate 150, while transistor 144 is situated in anN-well 152 with P-type source/drain 154 and its own gate 150. If contactholes 26 are irradiated by electron beam 130 alone, the P-N-P structureof PMOS transistor 144 will present a very high resistance to anyspecimen current that is generated, regardless of external bias.Therefore, it becomes difficult to make accurate measurements of thequality of contact holes 26 based on the specimen current.

To solve this problem, light source 160 irradiates structure 140 withbeam 162, which typically comprises visible, near infrared orultraviolet light. The photon energy of beam 162 is chosen so thatabsorption of the beam causes electron-hole pairs to be generated withinthe P-N junctions of transistors 142 and 144, as well as in P/N wellsand the bulk silicon of substrate 28. In silicon, for example, thefrequency of beam 162 is typically chosen to give photon energy in thevicinity of the bandgap (about 1.12 eV) or above. The presence of thephotoelectrons substantially increases the conductivity of PMOStransistor 144, so that the specimen current upon irradiation byelectron beam 130 can be readily measured. The photoelectrons may alsoenhance the measurement of specimen current from NMOS transistor 142, ifthe natural conductivity due to negatively-charged dielectric 130 isinsufficient.

Other applications of combined electromagnetic and charged particleirradiation in semiconductor wafer inspection will be apparent to thoseskilled in the art. For example, when electromagnetic energy is appliedduring SEM imaging, the contrast properties of circuit features may bealtered, thereby providing additional image information that would nototherwise be present.

Although the embodiments described hereinabove are directed particularlyto contact hole monitoring, the principles of the present invention mayalso be applied to other quality control tasks, such as measurement andmonitoring of other feature dimensions (particularly criticaldimensions) in the semiconductor wafer fabrication process. The methodsof the present invention provide an indication both of the width of suchfeatures and of the thickness of layers making up the features. Thesemethods can be adapted for use not only before metal deposition, as inthe embodiments described above, but also after metal deposition toinspect contacts, interconnects and metal lines for disconnects, shortcircuits and other defects.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. A method for process monitoring, comprising: receiving a samplehaving a first layer that is at least partially conductive and a secondlayer formed over the first layer, following production of contactopenings in the second layer by an etch process, the contact openingscomprising al plurality of test openings having different, respectivetransverse dimensions; directing a beam of charged particles toirradiate the test openings; measuring, in response to the beam, atleast one of a specimen current flowing through the first layer and atotal yield of electrons emitted from a surface of the sample, thusproducing an etch indicator signal; and analyzing the etch indicatorsignal as a function of the transverse dimensions of the test openingsso as to assess a characteristic of the etch process.
 2. The methodaccording to claim 1, wherein analyzing the etch indicator signalcomprises assessing a residual thickness of the dielectric layer at abottom of the test openings as a function of the transverse dimensions.3. The method according to claim 2, wherein the test openings comprise afirst opening having a first transverse dimension, and at least a secondopening having a second transverse dimension that is less than the firsttransverse dimension, and the method further comprises controlling theetch process, in response to the etch indicator signal, so that thefirst opening is sufficiently deep to reach the first layer, while atleast the second opening is not sufficiently deep to reach the firstlayer.
 4. The method according to claim 3, wherein the test openingsfurther comprise a third opening, having a third transverse dimensionintermediate the first and second transverse dimensions, and whereinanalyzing the etch indicator signal comprises detecting a potentialprocess defect when the etch indicator signal indicates that the thirdopening is not sufficiently deep to reach the first layer.
 5. The methodaccording to claim 2, wherein the sample has a barrier layer formedbetween the first and second layers, and wherein assessing the residualthickness comprises analyzing the etch indicator signal after etchingthe second layer in order to assess an integrity: of the barrier layer,and then analyzing the etch indicator signal after etching the barrierlayer.
 6. The method according to claim 1, wherein analyzing the etchindicator signal comprises assessing a critical dimension of a bottom ofthe test openings as a function of the transverse dimensions.
 7. Themethod according to claim 1, wherein analyzing the etch indicator signalcomprises measuring a beam current of the beam of charged particles, andanalyzing a ratio of the etch indicator signal to the beam current. 8.The method according to claim 1, wherein measuring at least one of thespecimen current and the total yield of the electrons comprisesmeasuring the total yield of the electrons emitted from the surface ofthe sample and further comprises measuring a primary current of thebeam, and taking a difference between the primary current and the totalyield to determine the etch indicator signal.
 9. The method according toclaim 1, wherein the plurality of test openings comprises multiplegroups of the test openings in respective test areas, which aredistributed in different locations across the sample, and whereindirecting the beam comprises positioning at least one of the beam andthe sample so as to irradiate each of at least two of the test areas inturn.
 10. The method according to claim 9, wherein analyzing the etchindicator signal comprises evaluating a variation of the etch indicatorsignal across the sample so as to assess a uniformity of the etchprocess.
 11. The method according to claim 1, wherein the plurality oftest openings comprise at least first and second arrays of testopenings, characterized by different, respective first and secondspacings between the test openings in the arrays.
 12. The methodaccording to claim 1, wherein directing the beam comprises irradiatingthe test openings along a beam axis that deviates substantially in anglefrom a normal to a surface of the sample.
 13. The method according toclaim 12, wherein the test openings have side walls and a bottom, andwherein irradiating the test openings comprises angling the beam so thatmore of the charged particles strike the side walls than strike thebottom.
 14. The method according to claim 1, wherein directing the beamcomprises operating the beam so as to precharge a surface of the samplein proximity to the test openings, so as to facilitate measurement ofthe specimen current.
 15. The method according to claim 14, whereinoperating the beam so as to precharge the surface causes electrons to beemitted from the surface, and comprising creating an electric field in avicinity of the surface so as to cause at least a portion of the emittedelectrons to return to the surface, thereby generating a negative precharge at the surface.
 16. The method according to claim 1, wherein thesample comprises a semiconductor wafer, and the contact openingscomprise at least one of contact holes, trenches and vias.
 17. Themethod according to claim 1, wherein receiving the sample comprisesreceiving the sample with a photoresist layer overlying the secondlayer, the photoresist layer having been used in etching the contactopenings, and wherein analyzing the etch indicator signal comprisesmonitoring the etch indicator signal while irradiating the test area,prior to removing the photoresist layer.
 18. The method according toclaim 17, and comprising, if the etch indicator signal indicates that aresidual thickness of the second layer at a bottom of one or more of thetest openings is greater than a predetermined limit, further etching thesecond layer using the photoresist layer so as to increase the depth.19. The method according to claim 1, wherein the sample comprises asemiconductor wafer, and wherein at least some of the contact openingsnot comprised in the plurality of test openings belong to multiplemicroelectronic circuits on the wafer, wherein the circuits areseparated by scribe lines, and the test openings are located or one ofthe scribe lines.
 20. The method according to claim 1, wherein analyzingthe etch indicator signal comprises detecting a residue within thecontact openings, and comprising irradiating the sample with the beam ofcharged particles so as to remove the residue.
 21. The method accordingto claim 1, wherein directing the beam comprises directing a pulsed beamof the charged particles to irradiate the test openings, and whereinmeasuring at least one of the specimen current and the total yield ofelectrons comprises measuring a time variation of the specimen currentby capacitive coupling to the sample.
 22. A method for processmonitoring, comprising: receiving a sample having a first layer that isat least partially conductive and a second layer formed over the firstlayer, following production of contact openings in the second layer byan etch process, the contact openings comprising at least first andsecond arrays of test openings, characterized by different, respectivefirst and second spacings between the test openings in the first andsecond arrays; directing a beam of charged particles to irradiate thearrays of test openings; measuring, in response to the beam, at leastone of a specimen current flowing through the first layer and a totalyield of electrons emitted from a surface of the sample, thus producingan etch indicator signal; and analyzing the etch indicator signal as afunction of the spacings of the arrays of the test openings so as toassess a characteristic of the etch process.
 23. The method according toclaim 22, wherein analyzing the etch indicator signal comprisesassessing a residual thickness of the dielectric layer at a bottom ofthe test openings as a function of the spacings.
 24. The methodaccording to claim 23, wherein the first spacing is substantiallygreater than the second spacing, and the method comprises controllingthe etch process, in response to the etch indicator signal, so that thetest openings in the first array are sufficiently deep to reach thefirst layer, while the test openings in the second array are notsufficiently deep to reach the first layer.
 25. The method according toclaim 23, wherein the sample has a barrier layer formed between thefirst and second layers, and wherein assessing the residual thicknesscomprises analyzing the etch indicator signal after etching the secondlayer in order to assess an integrity of the barrier layer, and thenanalyzing the etch indicator signal after etching the barrier layer. 26.The method according to claim 22, wherein analyzing the etch indicatorsignal comprises assessing a critical dimension of a bottom of the testopenings as a function of the transverse dimensions.
 27. The methodaccording to claim 22, wherein analyzing the etch indicator signalcomprises measuring a beam current of the beam of charged particles, andanalyzing a ratio of the etch indicator signal to the beam current. 28.The method according to claim 22, wherein measuring at least one of thespecimen current and the total yield of the electrons comprisesmeasuring the total yield of the electrons emitted from the surface ofthe sample and further comprises measuring a primary current of thebeam, and taking a difference between the primary current and the totalyield to determine the etch indicator signal.
 29. The method accordingto claim 22, wherein the test openings comprise multiple arrays of thetest openings having different, respective spacings in a plurality oftest areas, which are distributed in different locations across thesample, and wherein directing the beam comprises positioning at leastone of the beam and the sample so as to irradiate each of at least twoof the test areas in turn.
 30. The method according to claim 29, whereinanalyzing the etch indicator signal comprises evaluating a variation ofthe etch indicator signal across the sample so as to assess a uniformityof the etch process.
 31. The method according to claim 22, whereindirecting the beam comprises irradiating the test openings along a beamaxis that deviates substantially in angle from a normal to a surface ofthe sample.
 32. The method according to claim 31, wherein the testopenings have side walls and a bottom, and wherein irradiating the testopenings comprises angling the beam so that more of the chargedparticles strike the side walls than strike the bottom.
 33. The methodaccording to claim 22, wherein directing the beam comprises operatingthe beam so as to precharge a surface of the sample in proximity thetest openings, so as to facilitate measurement of the specimen current.34. The method according to claim 33, wherein operating the beam so asto precharge the surface causes electrons to be emitted from thesurface, and comprising creating an electric field in a vicinity of thesurface so as to cause at least a portion of the emitted electrons toreturn to the surface, thereby generating a negative precharge at thesurface.
 35. The method according to claim 22, wherein the samplecomprises a semiconductor wafer, and the contact openings comprise atleast one of contact holes, trenches and vias.
 36. The method accordingto claim 22, wherein receiving the sample comprises receiving the samplewith a photoresist layer overlying the second layer, the photoresistlayer having been used in etching the contact openings, and whereinanalyzing the etch indicator signal comprises monitoring the etchindicator signal while irradiating the test area, prior to removing thephotoresist layer.
 37. The method according to claim 36, and comprising,if the etch indicator signal indicates that the residual thickness ofthe second layer at the bottom of one or more of the test openings isless than a predetermined limit, further etching the second layer usingthe photoresist layer so as to increase the depth.
 38. The methodaccording to claim 22, wherein the sample comprises a semicond torwafer, and wherein at least some of the contact openings not comprisedin the plurality of test openings belong to multiple microelectroniccircuits on the wafer, wherein the circuits are separated by scribelines, and the test openings are located on one of the scribe lines. 39.The method according to claim 22, wherein analyzing the etch indicatorsignal comprises detecting a residue within the contact openings andcomprising irradiating the sample with the beam of charged particles soas to remove the residue.
 40. The method according to claim 22, whereindirecting the beam comprises directing a pulsed beam of the chargedparticles to irradiate the test openings, and wherein measuring at leastone of the specimen current and the total yield of electrons comprisesmeasuring a time variation of the specimen current by capacitivecoupling to the sample.
 41. A method for process monitoring of a samplehaving a first layer that is at least partially conductive and a secondlayer formed over the first layer, wherein contact openings are formedin the second layer by an etch process, the method comprising:determining, for a given set of characteristics of the contact openings,a threshold level of an etch indicator signal, which is produced bymeasuring at least one of a specimen current flowing through the firstlayer and a total yield of electrons emitted from a surface of thesample in response to irradiation of the contact openings by a beam ofcharged particles; directing the beam of charged particles to irradiateeach of a plurality of the contact openings that have the given set ofcharacteristics and are disposed at different, respective positions overa surface of the sample; determining, in response to the beam, the etchindicator signal produced at each of the respective positions of theplurality of the contact openings; and comparing the etch indicatorsignal produced at the respective positions to the threshold level so asto assess a characteristic of the etch process.
 42. The method accordingto claim 41, wherein comparing the etch indicator signal comprisesdetermining, if an absolute magnitude of the specimen current fallsbelow the threshold level by more than a predetermined margin, that atleast some of the contact openings are underetched.
 43. The methodaccording to claim 41, wherein determining the threshold level comprisesfinding the level of the etch indicator signal that corresponds toetching of the contact openings through the second layer to expose thefirst layer within the opening.
 44. The method according to claim 43,wherein finding the level comprises calibrating the threshold level in aprocedure performed on a test sample, for subsequent application inassessing the characteristic of the etch process performed on othersamples.
 45. The method according to claim 44, wherein calibrating thethreshold level comprises making measurements of the etch indicatorsignal generated by the test sample, and comparing the measurements toat least one of a cross-sectional profile of the contact openings in thetest sample and a conductivity of electrical contacts through thecontact openings in the test sample.
 46. The method according to claim43, wherein the sample has a barrier layer formed between the first andsecond layers, and wherein finding the level of the etch indicatorsignal comprises finding a first level that corresponds to etching ofthe contact openings through the second layer to expose the barrierlayer, and finding a second level that corresponds to etching of thecontact openings through the barrier layer to expose the first layerwithin the openings.
 47. The method according to claim 46, whereincomparing the etch indicator signal comprises analyzing the etchindicator signal after etching the second layer in order to assess anintegrity of the barrier layer, and then analyzing the etch indicatorsignal after etching the barrier layer in order to verify that at leastsome of the contact openings have been etched through the barrier layerto the first layer.
 48. The method according to claim 41, and comprisingevaluating a variation of the etch indicator signal across the sample soas to assess a uniformity of the etch process.
 49. The method accordingto claim 48, wherein evaluating the variation comprises signaling that aprocess fault has occurred if the variation of the etch indicator signalacross the sample is greater than a predetermined maximum.
 50. Themethod according to claim 41, wherein the sample comprises asemiconductor wafer, and wherein the contact openings comprise at leastone of contact holes, trenches and vias.
 51. The method according toclaim 41, wherein the sample has a photoresist layer overlying thesecond layer, the photoresist layer having been used in etching thecontact openings, and wherein measuring the etch indicator signalcomprises measuring the etch indicator signal prior to removing thephotoresist layer.
 52. The method according to claim 51, and comprising,if the etch indicator signal indicates that a depth of one or more ofthe contact openings is less than a predetermined limit, further etchingthe second layer using the photoresist layer so as to increase thedepth.
 53. The method according to claim 41, wherein determining theetch indicator signal comprises measuring a beam current of the beam ofcharged particles, and analyzing a ratio of at least one of the specimencurrent and the total yield of the electrons to the beam current. 54.The method according to claim 41, wherein directing the beam comprisesdirecting a pulsed beam of the charged particles to irradiate thesample, and wherein determining the etch indicator signal comprisesmeasuring a time variation of the specimen current by capacitivecoupling to the sample.
 55. A method for process monitoring of a samplehaving a first layer that is at least partially conductive and a secondlayer formed over the first layer, wherein contact openings are formedin the second layer by an etch process, the method comprising: directinga beam of charged particles to irradiate each of a plurality of theopenings that share a given set of characteristics and are disposed atdifferent, respective positions scross the sample; measuring at leastone of a specimen current flowing through the first layer and a totalyield of electrons emitted from a surface of the sample in response toirradiation of the contact openings by the beam of charged particles, usproducing an etch indicator signal as a function of the respectivepositions of the plurality of the openings; and evaluating a variationof the etch indicator signal across the sample so as to assess auniformity of the etch process.
 56. The method according to claim 55,wherein evaluating the variation comprises determining that a processfault has occurred if the variation of the etch indicator signal acrossthe sample is greater than a predetermined maximum.
 57. The methodaccording to claim 55, wherein the sample comprises a semiconductorwafer, and wherein the contact openings comprise at least one of contactholes, trenches and vias.
 58. The method according to claim 55, whereinevaluating the variation of the etch indicator signal comprisesmeasuring a beam current of the beam of charged particles, and analyzinga ratio of the etch indicator signal to the beam current.
 59. The methodaccording to claim 55, wherein directing the beam comprises directing apulsed beam of the charged particles to irradiate: the sample, andwherein measuring the specimen current comprises measuring a timevariation of the specimen current by capacitive coupling to the sample.60. Apparatus for monitoring a process applied to a sample having afirst layer that is at least partially conductive and a second layerformed over the first layer, contact openings having been created in thesecond layer by an etch process, the apparatus comprising: a teststation, comprising: a particle beam source, which is adapted to directa beam of charged particles to irradiate each of a plurality f thecontact openings that are disposed at different, respective positionsover a surface of the sample; and a current measuring device, which isadapted to produce an etch indicator signal by measuring, in response toirradiation of each of the plurality of the contact openings by the beamof charged particles, at least one of a specimen current flowing throughthe first layer and a total yield of electrons emitted from a surface ofthe sample; and a controller, which is adapted to store a calibratedthreshold level of the etch indicator signal for a given set ofproperties of the etch process, and to compare the respective etchindicator signal produced with respect to each of the plurality of thecontact openings to the threshold level so as to assess a characteristicof the etch process.
 61. The apparatus according to claim 60, whereinthe controller is adapted to determine, if an absolute magnitude of thespecimen current falls below the threshold level by more than apredetermined margin, that at least some of the contact openings areunderetched.
 62. The apparatus according to claim 61, wherein thethreshold level is calibrated by finding the level of the etch indicatorsignal that corresponds to etching of the contact openings through thesecond layer to expose the first layer within the opining.
 63. Theapparatus according to claim 62, wherein the threshold level iscalibrated in a procedure performed on a test sample, for subsequentapplication in assessing the characteristic the etch process performedon other samples.
 64. The apparatus according to claim 63, wherein thethreshold level is calibrated making measurements of the etch indicatorsignal generated by the test sample, and comparing the measurements toat least one of a cross-sectional profile he contact openings in thetest sample and a conductivity of electrical contacts made through thecontact openings in the test sample.
 65. The apparatus according toclaim 60, wherein the controller is further adapted to analyze avariation of the etch indicator signal across the sample so as to assessa uniformity of the etch process.
 66. The apparatus according to claim65, wherein the controller is adapted to signal that a process fault hasoccurred if the variation of the etch indicator signal across the sampleis greater than a predetermined maximum.
 67. The apparatus according toclaim 60, wherein the sample comprises a semiconductor wafer, andwherein the contact openings comprise at least one of contact holes,trenches and vias.
 68. The apparatus according to claim 60, wherein thesample has a photoresist layer overlying the second layer, which is usedin etching the contact openings, and wherein the test station is adaptedto measure the etch indicator signal prior to removing the photoresistlayer.
 69. The apparatus according to claim 68, and comprising an etchstation, which is adapted to form the contact openings in the secondlayer by the etch process, wherein the controller is adapted to causethe etch station to further etch the second layer using the photoresistlayer so as to increase a depth of one or more of the contact openings,if the etch indicator signal indicates that the depth is less than apredetermined limit.
 70. The apparatus according to claim 60, whereinthe current measuring device is further adapted to measure a beamcurrent of the: beam of charged particles, and wherein the control isadapted to analyze a ratio of at least one of the specimen current andthe total yield of the electrons to the beam current.
 71. The apparatusaccording to claim 60, wherein the particle beam source is adapted topulse the beam of the charged particles that irradiates the sample, andwherein the current measuring device is capacitively coupled to thesample so as to measure a time variation of the specimen current. 72.Theapparatus according to claim 60, wherein the sample has a barrier layerformed between the first and second layers, and wherein the apparatuscomprises an etch station, which is adapted to form the contact openingsin the second layer by the etch process and to etch the contact openingsthrough the barrier layer, and wherein the controller is adapted tostore first and second calibrated threshold levels of the etch indicatorsignal and to compare the respective etch indicator signal to the firstcalibrated threshold level after the etch station has etched the contactopenings through the second layer, and to the second calibratedthreshold level after the station has etched the contact openingsthrough the barrier layer.
 73. The apparatus according to claim 72,wherein the first calibrated old level corresponds to etching of thecontact openings through the second layer to expose the barrier layer,and the second calibrated threshold level ponds to etching of thecontact openings through the barrier layer to expose the first layerwithin the openings, and wherein the controller is adapted to comparethe etch indicator signal to the first calibrated threshold level afteretching the second layer in order to assess an integrity to the barrierlayer, and to compare the etch indicator signal to the second calibratedthreshold level etching of the barrier layer in order to verify at leastsome of the contact openings have been etched through the barrier layerto the first layer.
 74. Apparatus for monitoring a process applied to asample having a first layer that is at least partially conductive and asecond layer formed over the first layer, contact openings having beenformed in the second layer by an etch process, the apparatus comprising:a test station, which comprises: a particle beam source, which isadapted to direct a beam of charged particles to irradiate each of aplurality of the openings that are disposed at different, positionsacross the sample; and a current measuring device, which is adapted tomeasure at least a specimen current flowing through the layer and atotal yield of electrons emitted from a surface of the sample inresponse to irradiation of the contact openings by the beam of chargedparticles, thus producing an etch indicator signal as a function of therespective positions of the plurality of the openings; and a controller,which is adapted to evaluate a variation of the etch indicator signalacross the sample so as to assess a uniformity of the etch process. 75.The apparatus according to claim 74, wherein the controller is adaptedto determine that a process fault has occurred if the variation of theetch indicator signal across the sample is greater than a predeterminedmaximum.
 76. The apparatus according to claim 74, wherein the samplecomprises a semiconductor wafer, and wherein the contact openingscomprise at least one of contact holes, trenches and vias.
 77. Theapparatus according to claim 74, wherein the current measuring device isfurther adapted to measure a beam current of the beam of chargedparticles, and wherein the controller is adapted to analyze a ratio ofthe etch indicator signal to the beam current.
 78. The apparatusaccording to claim 74, wherein the particle beam source is adapted topulse the beam of the charged particles that irradiates the sample, andwherein the current measuring device is capacitively coupled to thesample so as to measure a time variation of the specimen current.